The present invention relates to a method of controlling the density of images produced by an electrophotographic image forming apparatus to a constant value.
Generally, an image forming apparatus implemented by an electrophotographic procedure such as an electrophotographic copier uses a photoconductive element having a photoconductive surface thereof. While a document is illuminated, a reflection from the document is focused onto the photoconductive element to electrostatically form a latent image representative of the document. A toner is electrostatically deposited on the latent image to produce a toner image. The toner image is transferred to a paper sheet or similar medium to complete a desired reproduction or copy. In this type of copier, stabilizing image density is one of most important conditions for insuring high image quality. Some different approaches have heretofore been proposed to stabilize image quality, as follows.
A first approach consists in forming a latent image of a reference density pattern, i.e., a black image having a reference density on a photoconductive element, sensing the amount of toner deposited on the latent image in terms of a reflectance by using an image density sensor (sometimes referred to as a P sensor hereinafter), comparing the resulted output of the sensor with a reference output representative of the reference density, and controlling the amount of toner to be supplied to a developer stored in a developing device in response to the result of comparison. A second approach is such that an electrometer detects the potential of a latent image formed on a photoconductive element and representative of a reference density pattern, and the charging, exposing and developing bias conditions are so controlled as to bring the potential to a predetermined adequate level. A third approach is practicable with a two-component developer which is a mixture of a toner and a carrier. Specifically, the third approach executes a sequence of steps of detecting the fluidity of the developer by a toner density sensor (sometimes referred to as an F sensor), determining the proportion of the toner and the carrier on the basis of the detected fluidity of the developer, and controlling the toner density in matching relation to the determined proportion.
The first to third conventional approaches outlined above have various problems left unsolved. Regarding the first approach, assume that the charge potential is lowered due to the fatigue of the photoconductive element, that the charge on the photoconductive element is locally disturbed by the deterioration of a charger or similar cause, or that the amount of light for illumination is not uniform due to the contamination and deterioration of optics used to form the latent image of the reference density pattern. Then, such an occurrence would be included in the control over the toner supply and would thereby cause an excessive amount of toner to be supplied to the developer. For example, when the charge deposited on the photoconductive element is made lower in an area where the latent image of the reference density pattern is to be formed than the other area due to local contamination of the charger, the detected density of the resultant toner image is lower than the actual density. As a result, the toner concentration of the developer is noticeably increased from the ordinary concentration, causing the toner to scatter around. Conversely, when the toner is not accurately supplied to the developer, the proportion of the carrier to the toner increases and, hence, the carrier is apt to mechanically to damage the photoconductive element by rubbing itself against the latter.
The second approach successfully stabilizes the potential of the latent image formed on the photoconductive element, but it has the following drawbacks. Specifically, assuming that the photoconductive element has been deteriorated due to fatigue, compensating the degradation of the charging characteristic ascribable to the fatigue by controlling the charging condition often lowers the optical sensitivity of the photoconductive element. Further, when the developing bias condition is controlled to control the image density, the contamination of the background on the photoconductive element and the image density act in a contrary relationship and thereby degrade the image quality, although an adequate developing potential may be achieved in the event of development.
Regarding the third approach, as the developer is deteriorated due to aging, for example, the amount of fatigued toner particles which do not contribute to development at all increases in the developer. This, coupled with the fact that the charging characteristic which is determined by the amount of charge deposited on the toner varies depending on the ambient conditions, makes it impractical to determine the image quality in terms of the toner concentration of the developer. It follows that the toner concentration of the developer which directly affects the image quality cannot be fixed. Further, it is difficult to achieve stable images because the amount of charge on the toner changes, as stated above. In addition, the third approach cannot adapt itself to the variation of the photoconductive element.
The first to third approaches stated above may be suitably combined to eliminate their drawbacks, as has been proposed. Specifically, the image density sensor or P sensor and the toner density sensor or F sensor particular to the first and third approaches, respectively, may be combined, as disclosed in Japanese Patent Laid-Open Publication (Kokai) No. 136667/1982 by way of example. Then, the control level (reference value) of the F sensor will be automatically changed in response to an output signal of the P sensor so as to control the image density. This kind of scheme provides images with stable density by compensating for the drawbacks particular to the individual sensors.
However, in the combination of P and F sensors, the reference value of the F sensor is unconditionally changed in response to the output of the P sensor. It is likely, therefore that when the characteristic of the developer is changed due to a change in temperature, humidity or similar ambient condition or when a particular kind of document image is used, the reference value of the F sensor greatly differs from the actual detected level. Specifically, the characteristics (density, amount of charge, and so forth) of the developer and photoconductive element are apt to vary temporarily over a substantial range when temperature, humidity or similar ambient condition sharply changes or depending on the conditions of use. If the P sensor senses the reflectance under such a condition, the reference value of the F sensor will be changed more than necessary or sharply. Then, the toner would be supplied to the developer abruptly in a great amount to increase the toner concentration or would not be supplied at all over a long period of time to reduce the toner concentration. This brings about various problems such as the scattering of toner particles, the abrupt change in the image density, and the damage to the developer. Another drawback with the prior art method is that since the F sensor is provided with a single reference value. Specifically, if the single reference value has been changed, a serviceman cannot see the degree to which the developer has been deteriorated at the time of inspection and, therefore, cannot predict the adequate time for maintenance. Since the upper and lower limits of developer density are determined beforehand machine by machine and cannot be changed later, it is impracticable to change them in matching relation to the developer at the time of replacement of the developer, for example, or to change them for compensating for irregularities among machines. For the reasons described above, it often occurs that the developer is replaced before it is deteriorated or is not replaced at all even after the deterioration.